The invention relates generally to ignition systems such as used with gas turbine engines. More particularly, the invention relates to temperature sensing and control for heated igniters used in such ignition systems.
Known ignition systems use many different types of igniter devices. Some ignition systems use spark type igniters, as are commonly used in the automotive industry. Spark igniters are also commonly used in aerospace applications. However, another type of igniter that is gaining more widespread use in many ignition systems is a glow plug, such as is described, for example, in U.S. Pat. Nos. 4,345,555, 4,582,981 and 4,825,658, the entire disclosures of which are fully incorporated herein by reference. Such devices are heated, such as, for example, by applying electrical energy to electrodes of the plug, to a temperature sufficient to initiate and/or to sustain combustion of a fuel/air mixture exposed to the plug. In some applications, a catalyst is applied to the outer surface of the heated element to further promote the combustion process.
In unthrottled combustion applications, such as, for example, combustion in a gas turbine engine, thermal runaway can occur at the glow plug. Thermal runaway may occur due to the net heat energy dissipated in and around the glow plug during operation. The glow plug is typically heated via electrical energy (existing systems rely on DC or AC currents to attain I.sup.2 R heating within the glowing element of the plug). In flow through type combustors prior to the occurrence of ignition the primary mode of heat dissipation for the glow plug is conductive cooling in a turbulent combustible flow. The volume, rate, and dynamic characteristics of this flow mandate that the power required to heat the glow plug to ignition temperatures (i.e. 800.degree. F.-1500.degree. F.) is in excess of the power required to destroy the element during no flow conditions, flow perturbations resulting in the development of catastrophic hot spots or sustained heating once ignition has occurred. If any of the aforementioned conditions occur, failure of the glow plug is eminent unless the heat source for the glow plug can be controlled via a fast stable temperature sensing/control system.
A critical portion of this system is the temperature sensor and signal conditioning system. Typically, the temperature of a glow plug is determined by either welding a thermocouple directly to a portion of the plug surface which will be representative of the plug's maximum surface temperature or by measuring and regulating the electrical current to the plug and assuming thermal stability exists at the plug surface. The first method of temperature measurement exhibits two major problems: A) the temperature sensed is the local temperature at the thermocouple weld site consequently undetectable hot spots can develop; and B) the extreme thermal cycling (i.e., Ambient to 1500 degrees F. per ignition cycle) and extended operation at the elevated temperatures maximize the mechanical fatigue at and around the weld site causing limited life. The primary problem with the second method (measuring and regulating the electrical current) is that the current required to heat the glow plug to ignition temperatures will vary over a wide range due to the cooling characteristics of the turbulent combustible flow discussed earlier. Regulating the current at a preset value will promote premature failure during dynamic operational conditions or require the glow plug and power source to be over designed by several orders of magnitude making them an impractical ignition system solutions.
It is desired, therefore, to detect the temperature of the heated element of the plug. In many applications, precise temperature measurements are not required, but rather only an approximate temperature range is needed.
The objective exists, therefore, for a sensor that can detect temperature of the heated element of an igniter such as a glow plug. The sensor should be capable of providing non-contact multipoint course temperature detection and be able to produce an output that can be used as a control signal for controlling the temperature of the plug, especially through pre-ignition and ignition conditions.